1. Field of the Invention
The present invention is directed to coating compositions having long pot lives, short dry times under ambient conditions and a reduced tendency to yellow when exposed to elevated temperatures in which the binder is based on aldimines and polyisocyanates containing uretdione groups which have improved compatibility with the aldimines.
2. Background of the Invention
Coating compositions which may be cured at room temperature are known. One-component coating compositions contain fully reacted polyurethanes as the binder. These compositions have the advantage that they are available as fully formulated systems which may be directly applied to suitable substrates without any preliminary steps except for mild stirring. Disadvantages of these systems are that large amounts of organic solvents are needed to reduce the viscosity of fully reacted, i.e., high molecular weight, polyurethanes. The coating compositions are cured by evaporation of the solvent which is objectionable from an environmental perspective. In addition, in order to solubilize the polyurethanes in organic solvents, they must be essentially linear polyurethanes. While such polyurethanes possess properties which are suitable for many applications, they do not provide certain properties, e.g., solvent resistance which may be obtained from crosslinked polyurethanes.
Two-component coating compositions are also known. These compositions come in two containers. The first contains a polyisocyanate, while the second contains an isocyanate-reactive component, generally a polyol. The components are not mixed until they are ready to be used. One advantage of these compositions is that because the components are not pre-reacted to form a high molecular weight polymer, a suitable processing viscosity can be achieved without the need for large amounts of organic solvents. In addition, higher functional components can be used to obtain highly crosslinked coatings which possess properties which surpass those possessed by one-component coatings.
The disadvantages of these compositions is that they cannot be applied without a preliminary mixing step in which it is critical that the components are mixed in the right proportions. In addition, special metering and mixing equipment is needed to conduct this process on a commercial scale. If the components are mixed in the wrong proportions, then the properties of the resulting coatings can be substantially affected. In addition, after the components are mixed they must be used in a timely fashion. If not, they continue to react until an unusable solid is obtained.
Coating compositions which possess the advantages of the known one- and two-component coating compositions without possessing their disadvantages may be prepared by blending the polyisocyanate adducts such as trimers and biurets with certain aldimines. However, coatings prepared from these known components are unacceptable due to their appearance. Further improvements are needed in clarity, gloss and distinctness of image (DOI)in order for these compositions to be useful in coating applications. These properties are directly related to the compatibility between the polyisocyanate and the aldimine.
In addition, coatings prepared from coating compositions containing polyisocyanates and aldimines have a tendency to yellow upon exposure to elevated temperatures. This yellowness may be caused during curing at elevated temperatures or it may be caused when a coating, cured at ambient temperature, is subsequently exposed to elevated temperatures, e.g., coatings which are exposed to direct sunlight.
It is an object of the present invention to improve the clarity, gloss and DOI and resistance to yellowing of coating compositions based on polyisocyanates and aldimines. This object may be achieved with coating compositions containing, as binder, aldimines and polyisocyanates containing uretdione groups, optionally in admixture with other polyisocyanates such as polyisocyanates containing isocyanurate groups.
U.S. Pat. Nos. 3,420,800 and 3,567,692 disclose coating compositions containing polyisocyanates and either aldimines or ketimines. However, these patents do not teach that the compatibility of the binder components can be improved by the use of polyisocyanates according to the present invention. It has previously been disclosed in copending application, U.S. Ser. No. 08/171,281, that the compatibility of polyisocyanates with aldimines may be improved by using polyisocyanates containing allophanate groups derived from monoalcohols. However, there is no suggestion that similar improvements in clarity could also be obtained with polyisocyanates containing uretdione groups or that these latter polyisocyanates also result in a coating with improved resistance to yellowing at elevated temperatures.